Fishing at Home
If the idea of going fishing for real doesn't appeal to you or isn't possible, you can always create your own fishing environment at home. To do this, let's set up a game at home similar to those found at many fairs, where you're given a rod and need to fish for a little prize.
Implementing the idea is simple: just create a basic fishing rod (a stick, a string, and a hook) and prepare a place where your partner can fish for little gifts (the bathtub or a small inflatable pool like the ones sold for kids at the beach). Once we have this, we'll prepare many small trinkets that can be fished with varying degrees of ease, so the game lasts as long as possible.
The trinkets can be of all kinds: from a chocolate coin to a ring, or even a love note... To add more fun to the game, it's recommended to put them in small, opaque plastic boxes (a small lunchbox might work), so you never know what you're going to fish. Each box should have a solid ring attached to allow for fishing (a wire will do... and if you're feeling crafty, you can shape the ring into a heart ;)).
Regardless of the fishing success, you can be sure you'll surprise your partner a lot with this idea.